On the first 3 exams of his geometry class, Luis got an average score of 84. What does he need on the next exam to have an overall average of 86?
Answer: Let his score on the next exam be $x$ The sum of all of his scores is then $3 \cdot 84 + x$ The same sum must also be equal to $4 \cdot 86$ Solve: $x = 4 \cdot 86 - 3 \cdot 84 = 92$.